BCMA (CD269 or TNFRSF17) is a member of the TNF receptor superfamily. It is a non-glycosylated integral membrane receptor for the ligands BAFF and APRIL. BCMA's ligands can also bind additional receptors: TACI (Transmembrane Activator and Calcium modulator and cyclophilin ligand Interactor), which binds APRIL and BAFF; as well as BAFF-R (BAFF Receptor or BR3), which shows restricted but high affinity for BAFF. Together, these receptors and their corresponding ligands regulate different aspects of humoral immunity, B-cell development and homeostasis.
BCMA's expression is typically restricted to the B-cell lineage and is reported to increase in terminal B-cell differentiation. BCMA is expressed by human plasma blasts, plasma cells from tonsils, spleen and bone marrow, but also by tonsillar memory B cells and by germinal centre B cells, which have a TACI-BAFFR low phenotype (Darce et al, 2007). BCMA is virtually absent on naïve and memory B-cells (Novak et al., 2004a and b). The BCMA antigen is expressed on the cell surface so is accessible to the antibody, but is also expressed in the golgi. As suggested by its expression profile, BCMA signalling, typically linked with B-cell survival and proliferation, is important in the late stages of B-cell differentiation, as well as the survival of long lived bone marrow plasma cells (O'Connor et al., 2004) and plasmablasts (Avery et al., 2003). Furthermore, as BCMA binds APRIL with high affinity, the BCMA-APRIL signalling axis is suggested to predominate at the later stages of B-cell differentiation, perhaps being the most physiologically relevant interaction.
Multiple Myeloma (MM) is a clonal B-cell malignancy that occurs in multiple sites within the bone marrow before spreading to the circulation; either de novo, or as a progression from monoclonal gammopathy of undetermined significance (MGUS). It is commonly characterised by increases in paraprotein and osteoclast activity, as well as hypercalcaemia, cytopenia, renal dysfunction, hyperviscosity and peripheral neuropathy. Decreases in both normal antibody levels and numbers of neutrophils are also common, leading to a life threatening susceptibility to infection. BCMA has been implicated in the growth and survival of myeloma cell lines in vitro (Novak et al., 2004a and b; Moreaux et al., 2004).
BCMA expression (both transcript and protein) is reported to correlate with disease progression in MM. Using Affymetrix microarrays, it was demonstrated that the TACI and BCMA genes were over-expressed in Multiple Myeloma Cells (MMC) compared with their normal counterparts (Moreaux et al, 2004). Gene expression analysis has been used to compare human myeloma cells with purified plasma cells from patients with MGUS and from normal bone marrow as well as with primary tumour cells from B-cell lineage leukaemias (Bellucci et al, 2005). The BCMA gene was highly expressed in all myeloma samples. Although purified plasma cells from patients with MGUS had lower expression of BCMA, there was no significant difference when compared with the expression found in normal plasma cells or myeloma cells. In contrast, BCMA expression was significantly lower in B-cell Chronic Lymphocytic Leukaemia (CLL), pre-B Acute Lymphocytic Leukaemia (ALL) and T-cell ALL (T-ALL). Mouse models that transgenically over-express BAFF or APRIL have a significant increase in B-cell lymphomas (Batten et al., 2004-BAFF; Planelles et al., 2004-APRIL). In humans, excess BAFF and APRIL have been detected in the sera and micro-environments of patients with a number of B-cell malignancies, as well as other B-cell disorders.
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